1. Field of Invention
The present invention relates to a method of driving an electro-optical apparatus, which is suitable for use in driving an electro-optical apparatus, a drive circuit for an electro-optical apparatus, an electro-optical apparatus, and an electronic apparatus.
2. Description of Related Art
Electro-optical apparatuses, for example, liquid crystal display apparatuses which use liquid crystal as an electro-optical material, are widely used in display units of various information processing apparatuses and liquid crystal television sets as an alternative display device to cathode ray tubes (CRT). A conventional electro-optical apparatus is constructed, for example, as follows. That is, the conventional electro-optical apparatus includes a device substrate on which pixel electrodes arranged in a matrix, and switching elements, such as TFTs (Thin Film Transistors) connected to the pixel electrodes, are formed; an opposing substrate on which an opposing electrode is formed opposing the pixel electrodes; and liquid crystal, as an electro-optical material, filled between the substrates.
In this structure, when a scanning signal is applied to the switching elements via scanning lines, the switching elements are turned on. When an image signal of a voltage in accordance with the gray scale level is applied to the pixel electrodes via data lines while the electrical connection is on, a charge corresponding to the voltage of the image signal is stored in the liquid crystal layer between the pixel electrodes and the opposing electrode. Even if the switching elements are turned off after the charge has been stored, the charge stored in the liquid crystal layer is maintained due to the capacitance of the liquid crystal layer and storage capacitance. When the charge stored is controlled in accordance with the gray scale level by driving the switching elements, the orientation of the liquid crystal changes on a pixel-by-pixel basis, the gray scale level thus changing on a pixel-by-pixel basis, whereby gray scale display is enabled.
At this time, the charge has to be stored in the liquid crystal layer of each of the pixels during only a short period, a time division multiplexed driving, in which the scanning lines and the data lines are grouped for a plurality of pixels, is enabled, by an arrangement such that a scanning line drive circuit first selects each of the scanning lines sequentially, a data line drive circuit next selects each of the data lines sequentially during the period when the scanning lines are selected, and an image signal of a voltage in accordance with the gray scale level is then sampled to selected data lines.
However, the image signal supplied to the data lines has a voltage corresponding to the gray scale, i.e., is an analog signal. Thus, in the peripheral circuitry of the electro-optical apparatus, a D/A conversion circuit, op amps, etc. are required, incurring higher cost of the overall apparatus. In addition, the display may not be uniform due to non-uniformity of the characteristics of the D/A conversion circuit, op amp, etc. and various line resistance, inhibiting display of a high quality, which becomes particularly prominent when the display requires a high resolution.
Furthermore, with regard to the electro-optical material, such as liquid crystal, the relationship between the voltage applied and the transmissivity varies depending on the type of the electro-optical material. Thus, it is desired that the drive circuit for driving the electro-optical apparatus be of the general-purpose type which is compatible with various electro-optical apparatuses.
In view of the above situation, the applicant has developed a technique to divide a single frame into a plurality of subfields, and turn on and off each of the pixels in each of the subfields. In accordance with this technique, the voltage applied when the pixels are turned on and off within a subfield is constant regardless of the gray scale level, and the gray scale level of the pixels is determined by the duty ratio (or effective voltage value) of the pixels within a single frame in the ON state.
When the gray scale level of the electro-optical apparatus is observed while changing the duty ratio in a range of 0 to 100%, a range exists near the duty ratio 0% or 100% in which the gray scale level does not change even though the duty ratio changes. The manner in which the range is generated varies depending on the composition of the liquid crystal; the range may be generated only near the duty ratio 0%, only near 100%, or both. Thus, corresponding to the ranges in which the gray scale level does not change, some subfields exist which are always set to on or off regardless of the specified gray scale level.
When the ON/OFF state of each of the pixels is changed at a boundary of subfields, storage capacitance, etc. is charged and discharged. Thus, during the period of charging and discharging, power consumption in the electro-optical apparatus and the drive circuit thereof is larger compared with other periods. Because the number of subfields increases as the number of gray scale levels in the electro-optical apparatuses increases, power consumption increases in accordance with the number of gray scale levels. For the same reason, power consumption associated with the signal lines and the scanning lines also increases.
However, even if a large number of gray scale levels is required in the electro-optical apparatus, it does not mean that display with the large number of gray scale levels is always required. For example, when a cellular phone is in a wait mode or a personal computer is in a power-saving mode, an electro-optical apparatus (e.g. a liquid crystal display) incorporated therein only requires simple display, and electric power is wasted if the large number of gray scale levels is maintained in such situations.